Sondrestrom incoherent scatter radar observations during the Lower Thermosphere Coupling Study: September 21–26, 1987

Abstract

The incoherent scatter radar located at S0ndre Str0m0ord, Greenland, obtained E and F region measurements during the first Lower Thermosphere Coupling Study (LTCS 1), September 21-26, 1987. Lower thermospheric neutral winds deduced from these measurements show that the neutral dynamics are influenced by both tidal oscillations and magnetospheric forcing. During an interval which was relatively quiet geomagnetically, September 23-24, a semidiurnal oscillation dominated the neutral motion. The model equinox tidal amplitudes and phases of Forbes (1982) for the diurnal tide are roughly in agreement with the observed diurnal oscillation for the first four days of the experiment. Vertical variations in the observed diurnal phases are consistent with the results of Forbes and Hagan (1988) and may provide evidence of dissipation of the propagating (1, 1) tidal mode. The semidiurnal motion observed during this period is not well represented by the recent theoretical results for the amplitude and phase of the semidiurnal tide (Forbes and Vial, this issue). Neutral winds obtained during a geomagnetically active interval, September 25-26, displayed a flow pattern that was significantly distorted from that observed during the preceding, relatively quiet interval. Although the changes in the zonal winds throughout this active interval were consistent with the direction of the ion drag force at 115 km and above, the variations in the meridional winds suggest that other forces, such as pressure gradients driven by Joule heating, need to be considered to explain the observations. ß , ß . ß , ß , ß ß ß ß ß ß ß -0.02

Studies of the E-Region Neutral Wind in the Auroral Ionosphere Using Two Long-Run Data.

“…We, thus, compare our results with those of Johnson (1991) rather than Johnson and Virdi (1991). Furthermore, Fesen and Roble (1991) used the National Center for Atmospheric Research Thermosphere-Ionosphere General Circulation Model (TIGCM) to simulate IS radar observations during the LTCS 1 campaign, and made comparisons of mean winds as well as diurnal and semidiurnal components with those of Johnson (1991). It is also worthwhile to compare the September results with the model predictions obtained for the Sondre Stromfjord data.…”

“…Johnson (1991) has derived the mean wind as well as tidal components for a four-day interval starting at 10 (UT) on September 21, and the interval is nearly equivalent to that of the September experiment with only two-hour difference. We, thus, compare our results with those of Johnson (1991) rather than Johnson and Virdi (1991). Furthermore, Fesen and Roble (1991) used the National Center for Atmospheric Research Thermosphere-Ionosphere General Circulation Model (TIGCM) to simulate IS radar observations during the LTCS 1 campaign, and made comparisons of mean winds as well as diurnal and semidiurnal components with those of Johnson (1991).…”

“…In our work we use MSIS-86 model (Hedin, 1987) for neutral densities and the formula by Schunk and Walker (1973) for a derivation of ion-neutral collision frequency. Both are also used in the studies by Johnson (1991) and Johnson and Virdi (1991) for the Sondre Stromfjord data, and Brekke et al (1994) and Nozawa and Brekke (1995) of averaged neutral winds in quiet-and disturbed-times, respectively. Comparison of mean winds as well as tidal components with the Sondre Stromfjord results by Johnson (1991) will be given in Subsection 4.2.…”

“…In this work we use the same neutral density model (Hedin, 1987) and ion-neutral collision frequency formulation (Schunk and Walker, 1973) as Johnson and Virdi (1991) and Johnson (1991) used for the Sondre Stromfjord data. Johnson (1991) has derived the mean wind as well as tidal components for a four-day interval starting at 10 (UT) on September 21, and the interval is nearly equivalent to that of the September experiment with only two-hour difference. We, thus, compare our results with those of Johnson (1991) rather than Johnson and Virdi (1991).…”

“…Usually the tidal components at a particular height are derived by the method of a least squares fit (LSF). The fit is often made by determining the mean wind and amplitude and phase of one or more tidal components such as the diurnal, semidiurnal and shorter period tides; this has been done in the works of Johnson et al (1987), Williams and Virdi (1989), Manson et al (1990Manson et al ( , 1992, Johnson (1991), Johnson and Virdi (1991), Kunitake and Schlegel (1991), Virdi and Williams (1993), and Kofman et al (1996). Brekke et al (1994) introduced the method of Fast Fourier Transform (FFT; discrete Fourier transform) for spectral analysis of averaged 24-hour data sets of the neutral wind velocities at different E-region heights.…”